Lithium-ion secondary battery

ABSTRACT

A lithium-ion secondary battery  100  includes a positive electrode current collector  221  and a porous positive electrode active material layer  223  retained by the positive electrode current collector  221 . The positive electrode active material layer  223  contains, for example, positive electrode active material particles  610 , an electrically conductive material  620 , and a binder  630 . In this lithium-ion secondary battery  100 , the positive electrode active material particles  610  have a shell portion  612  constituted by a lithium transition metal oxide, a hollow portion  614  formed inside the shell portion  612 , and a through hole  616  penetrating the shell portion  612 . In the lithium-ion secondary battery  100 , in the positive electrode active material layer  223  on average, the hollow portion  614  accounts for 23% or higher of an apparent sectional area of the positive electrode active material particles  610 . In addition, a thickness of the shell portion  612  in the positive electrode active material layer  223  on average is 2.2 μm or less.

TECHNICAL FIELD

The present invention relates to a lithium-ion secondary battery. In thepresent description, “lithium-ion secondary battery” refers to asecondary battery which uses lithium ions as electrolyte ions and inwhich charging and discharging are realized by the transfer of chargesaccompanying lithium ions between a positive electrode and a negativeelectrode. In the present description, “secondary battery” includesrepetitively chargeable storage devices in general.

BACKGROUND ART

For example, Japanese Patent Publication No. 4096754 discloses apositive electrode active material of a lithium-ion secondary battery.

The positive electrode active material disclosed in the literature abovecontains a lithium-nickel complex oxide with a layered structure. Inthis case, the lithium-nickel complex oxide with a layered structure isobtained by calcining a raw material mixture composed of a heat-treatedproduct of a coprecipitate obtained by precipitating sodium hydroxide ona water-based solution containing cobalt ions, nickel ions, or aluminumions at a predetermined composition ratio and a lithium compound. Thelithium-nickel complex oxide is represented by the following generalformula. General formula: Li_(k)Ni_(m)Co_(p)Al_((1-m-p))O_(r) (where k,m, p, and r respectively satisfy 0.95≦k≦1.10, 0.1≦m≦0.9, 0.1≦p≦0.9, and1.8≦r≦2.2), and Ni/Co (molar ratio) is any of 2.33, 3.0, 3.25, and 3.5.The lithium-nickel complex oxide disclosed in the literature above is ahollow particle comprising an outer shell portion on the outside and aspace portion inside the outer shell portion. In addition, a ratio of asurface area of the space portion to a total surface area of the outershell portion and the space portion in a processed profile of the hollowparticle is within a range of 7% to 16%.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Publication No. 4096754

SUMMARY OF INVENTION

A vehicle in which wheels are driven by an electric motor such asco-called hybrid vehicles (including plug-in hybrid vehicles) andelectric cars is capable of running by only using power stored in abattery. An output of a battery tends to decline as an amount of chargedecreases. To ensure that a vehicle runs stably, the battery isdesirably used within a predetermined amount-of-charge range. If thebattery mounted on this vehicle is capable of producing a requiredoutput even at a low amount of charge (even when the amount of charge islow), a traveling performance of a hybrid vehicle, an electrical car, orthe like can be improved. In addition, if a required output can beproduced even at a low amount of charge (even when the amount of chargeis low), the number of batteries used to secure a necessary amount ofenergy can be reduced and cost reduction can be achieved.

A lithium-ion secondary battery proposed by the present inventors has acurrent collector and a porous positive electrode active material layerwhich is retained by the current collector and which contains positiveelectrode active material particles, an electrically conductivematerial, and a binder. The positive electrode active material particleshave a shell portion constituted by a lithium transition metal oxide, ahollow portion formed inside the shell portion, and a through hole thatpenetrates the shell portion. In addition, in the positive electrodeactive material layer on average, the hollow portion accounts for 23% orhigher of an apparent sectional area of the positive electrode activematerial particles, and a thickness of the shell portion in the positiveelectrode active material layer on average is 2.2 μm or less. In thiscase, on any cross section of the positive electrode active materiallayer, a thickness of the shell portion at any position on an innersurface of the shell portion is defined as a shortest distance from thearbitrary position on the inner surface of the shell portion to an outersurface of the shell portion.

Generally, since a lithium ion concentration in the positive electrodeactive material layer increases significantly at a low amount of charge,diffusion of ions to the inside of the positive electrode activematerial during discharging is subjected to rate limitation. With thelithium-ion secondary battery according to the present invention, in thepositive electrode active material layer on average, the ratio of thehollow portion among the apparent sectional area of the positiveelectrode active material particles is 23% or higher, the positiveelectrode active material particles have a through hole penetrating theshell portion, and a thickness of the shell portion of the positiveelectrode active material particles is extremely thin (in this case, 2.2μm or less). Therefore, lithium ions diffuse rapidly into the shellportion (inside of the active material) of the positive electrode activematerial particles. As a result, the lithium-ion secondary battery canstably produce high output even when the amount of charge is low.

Furthermore, in the positive electrode active material layer on average,the thickness of the shell portion may be 0.05 μm or more. Accordingly,necessary durability is secured for the positive electrode activematerial particles and the performance of the lithium-ion secondarybattery stabilizes.

Moreover, the lithium transition metal oxide constituting the shellportion of the positive electrode active material particles may be acompound which has a layered structure and which contains nickel as aconstituent element. This lithium transition metal oxide may be, forexample, a compound which has a layered structure and which containsnickel, cobalt, and manganese as constituent elements. Alternatively,the lithium transition metal oxide may be a compound which has a layeredstructure and which is expressed asLi_(1+x)Ni_(y)Co_(z)Mn_((1-y-z))M_(γ)O₂, where 0≦x≦0.2, 0.1<y<0.9,0.1<z<0.4, M denotes an additive, and 0≦γ≦0.01. Furthermore, M as theadditive may be at least one additive selected from the group consistingof Zr, W, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, B, and F.

Furthermore, the positive electrode active material particles may bepositive electrode active material particles produced by a productionmethod comprising: a raw material hydroxide formation step of supplyingammonium ions to an aqueous solution of a transition metal compound, andprecipitating particles of the transition metal hydroxide from theaqueous solution; a mixing step of mixing the transition metal hydroxidewith a lithium compound to prepare an unfired mixture; and a calciningstep of calcining the mixture to obtain the active material particles.Moreover, in this case, the aqueous solution may contain at least onetransition metal element that composes the lithium transition metaloxide.

In addition, the raw material hydroxide formation step may include anucleation stage in which the transition metal hydroxide is precipitatedfrom the aqueous solution and a particle growth stage in which thetransition metal hydroxide is grown in a state where a pH of the aqueoussolution is lowered from the nucleation stage. In this case, the pH ofthe aqueous solution in the nucleation stage may be 12 to 13, and the pHof the aqueous solution in the particle growth stage may be 11 or higherand lower than 12. Furthermore, an ammonium ion concentration in theaqueous solution in the nucleation stage may be 20 g/L or lower, and theammonium ion concentration in the aqueous solution in the particlegrowth stage may be 10 g/L or lower. Moreover, the ammonium ionconcentration of the aqueous solution in the nucleation stage and theparticle growth stage may be 3 g/L or higher. Accordingly, positiveelectrode active material particles having a thin shell portion, aspacious hollow portion, and having a through hole can be obtained morestably.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing an example of a structure of a lithium-ionsecondary battery;

FIG. 2 is a diagram showing a wound electrode body of a lithium-ionsecondary battery;

FIG. 3 is a sectional view showing a cross-section taken along lineIII-III in FIG. 2;

FIG. 4 is a sectional view showing a structure of a positive electrodeactive material layer;

FIG. 5 is a sectional view showing a structure of a negative electrodeactive material layer;

FIG. 6 is a side view showing a welding location of an uncoated portionand an electrode terminal of a wound electrode body;

FIG. 7 is a diagram schematically showing a state during charging of alithium-ion secondary battery;

FIG. 8 is a diagram schematically showing a state during discharging ofa lithium-ion secondary battery;

FIG. 9 is a diagram showing positive electrode active materialparticles;

FIG. 10 is a graph exemplifying an approximate curve when calculatingoutput characteristics 1;

FIG. 11 is a graph exemplifying an approximate curve when calculatingoutput characteristics 2; and

FIG. 12 is a diagram showing a vehicle mounted with a secondary battery.

DESCRIPTION OF EMBODIMENTS

First, an example of a structure of a lithium-ion secondary battery willbe described. Subsequently, while referring to this structure example asappropriate, a lithium-ion secondary battery according to an embodimentof the present invention will be described. Members and portions thatproduce same effects are denoted by same reference characters asappropriate. In addition, it will be recognized that the respectivedrawings have been schematically rendered and therefore may notnecessarily reflect actual elements shown. The respective drawingsmerely show one example and do not limit the present invention theretounless specifically mentioned otherwise.

FIG. 1 shows a lithium-ion secondary battery 100. As shown in FIG. 1,the lithium-ion secondary battery 100 comprises a wound electrode body200 and a battery case 300. FIG. 2 is a diagram showing the woundelectrode body 200. FIG. 3 shows a cross-section taken along lineIII-III in FIG. 2.

As shown in FIG. 2, the wound electrode body 200 comprises a positiveelectrode sheet 220, a negative electrode sheet 240, and separators 262and 264. The positive electrode sheet 220, the negative electrode sheet240, and the separators 262 and 264 are respectively band-like sheetmaterials.

<Positive Electrode Sheet 220>

The positive electrode sheet 220 comprises a band-like positiveelectrode current collector 221 and a positive electrode active materiallayer 223. A metallic foil suitable for a positive electrode may be usedas the positive electrode current collector 221. For example, aband-like aluminum foil having a predetermined width and a thickness ofapproximately 15 μm can be used as the positive electrode currentcollector 221. An uncoated portion 222 is set along one width-directionedge of the positive electrode current collector 221. In the illustratedexample, as shown in FIG. 3, with the exception of the uncoated portion222 set on the positive electrode current collector 221, the positiveelectrode active material layer 223 is retained on both surfaces of thepositive electrode current collector 221. The positive electrode activematerial layer 223 contains positive electrode active material. Thepositive electrode active material layer 223 is formed by coating apositive electrode mixture containing the positive electrode activematerial onto the positive electrode current collector 221.

<Positive Electrode Active Material Layer 223 and Positive ElectrodeActive Material Particles 610>

FIG. 4 is a sectional view of the positive electrode sheet 220.Moreover, in FIG. 4, positive electrode active material particles 610,an electrically conductive material 620, and a binder 630 in thepositive electrode active material layer 223 are schematically depictedenlarged so as to clarify the structure of the positive electrode activematerial layer 223. As shown in FIG. 4, the positive electrode activematerial layer 223 contains the positive electrode active materialparticles 610, the electrically conductive material 620, and the binder630.

A material used as a positive electrode active material of a lithium-ionsecondary battery can be used as the positive electrode active material610. Examples of a positive electrode active material 610 includelithium transition metal oxides such as LiNiCoMnO₂(lithium-nickel-cobalt-manganese complex oxide), LiNiO₂ (lithiumnickelate), LiCoO₂ (lithium cobaltate), LiMn₂O₄ (lithium manganate), andLiFePO₄ (iron lithium phosphate). For example, LiMn₂O₄ has a spinelstructure. In addition, LiNiO₂ and LiCoO₂ have a layered evaporiticstructure. Furthermore, for example, LiFePO₄ has an olivine structure.LiFePO₄ having an olivine structure includes, for example, particles inthe order of nanometers. In addition, LiFePO₄ having an olivinestructure can be further coated by a carbon film.

<Electrically Conductive Material 620>

Examples of the electrically conductive material 620 include carbonmaterials such as carbon powders and carbon fibers. One type of materialselected from such electrically conductive materials may be used aloneor two or more types may be used in combination. Examples of carbonpowders that can be used include various types of carbon black (such asacetylene black, oil furnace black, graphitized carbon black, carbonblack, graphite, and Ketjen black) and graphite powder.

<Binder 630>

In addition, the binder 630 binds respective the positive electrodeactive material particles 610 and the particles of the electricallyconductive material 620 contained in the positive electrode activematerial layer 223 with each other, and binds these particles and thepositive electrode current collector 221 with each other. For thisbinder 630, a polymer can be used which is dissolvable or dispersible inthe solvent used. For example, in a positive electrode mixturecomposition that uses an aqueous solvent, a water-soluble orwater-dispersible polymer can be used favorably, examples of whichinclude: cellulose-based polymers (such as carboxymethyl cellulose (CMC)and hydroxypropyl methyl cellulose (HPMC)); fluorine-based resins (forexample, polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), andtetrafluoroethylene-hexafluoropropylene copolymer (FEP)); and rubbers(such as vinyl acetate copolymers, styrene butadiene copolymers (SBR),and acrylic acid-modified SBR resins (SBR latex)). In addition, in apositive electrode mixture composition that uses a non-aqueous solvent,polymers (such as polyvinylidene fluoride (PVDF), polyvinylidenechloride (PVDC), and polyacrylonitrile (PAN)) can favorably be used.

<Thickener, Solvent>

The positive electrode active material layer 223 is formed by, forexample, preparing a positive electrode mixture in which the positiveelectrode active material particles 610 and the electrically conductivematerial 620 described above are mixed into a solvent to a paste form(slurry), coating the positive electrode current collector 221 with thepositive electrode mixture, and subsequently performing drying androlling. In doing so, any aqueous solvent or non-aqueous solvent can beused for the solvent of the positive electrode mixture. A preferableexample of a non-aqueous solvent is N-methyl-2-pyrrolidone (NMP). Theabove-mentioned polymer materials exemplified as the binder 630 can alsobe used for the purpose of demonstrating a function as a thickener oranother additive of the positive electrode mixture in addition tofunctioning as a binder.

A weight ratio of the positive electrode active material in the entirepositive electrode mixture is favorably approximately 50% by weight ormore (and typically 50 to 95% by weight), and normally the ratio is morefavorably approximately 70 to 95% by weight (for example, 75 to 90% byweight). In addition, the ratio of the electrically conductive materialin the entire positive electrode mixture can favorably be, for example,approximately 2 to 20% by weight, and normally the ratio is favorablyapproximately 2 to 15% by weight. In a composition that uses a binder,the ratio of the binder in the entire positive electrode mixture can be,for example, approximately 1 to 10% by weight, and normally the ratio isfavorably approximately 2 to 5% by weight.

<Negative Electrode Sheet 240>

As shown in FIG. 2, the negative electrode sheet 240 comprises aband-like negative electrode current collector 241 and a negativeelectrode active material layer 243. A metallic foil suitable for anegative electrode may be preferably used as the negative electrodecurrent collector 241. A band-like copper foil having a predeterminedwidth and a thickness of approximately 10 μm is used as the negativeelectrode current collector 241. An uncoated portion 242 is set alongone width-direction edge of the negative electrode current collector241. With the exception of the uncoated portion 242 set on the negativeelectrode current collector 241, the negative electrode active materiallayer 243 is formed on both surfaces of the negative electrode currentcollector 241. The negative electrode active material layer 243 isretained by the negative electrode current collector 241 and at leastcontains negative electrode active material. For the negative electrodeactive material layer 243, a negative electrode mixture containing thenegative electrode active material is coated on the negative electrodecurrent collector 241.

<Negative Electrode Active Material Layer 243>

FIG. 5 is a sectional view of the negative electrode sheet 240 of thelithium-ion secondary battery 100. As shown in FIG. 5, the negativeelectrode active material layer 243 contains a negative electrode activematerial 710, a thickener (not shown), a binder 730, and the like. InFIG. 5, the negative electrode active material 710 and the binder 730 inthe negative electrode active material layer 243 are schematicallydepicted enlarged so as to clarify the structure of the negativeelectrode active material layer 243.

<Negative Electrode Active Material>

Furthermore, one type or two or more types of materials conventionallyused in lithium-ion secondary batteries can be used without particularlimitation for the negative electrode active material 710. Examples ofthese materials include particulate carbon materials (carbon powder)containing a graphite structure (a layered structure) in at least aportion thereof. More specifically, the negative electrode activematerial may be, for example, natural graphite, natural graphite coatedwith an amorphous carbon material, a carbon material having a graphiticstructure (graphite), a carbon material having a non-graphitizablecarbonaceous structure (hard carbon), a carbon material having agraphitizable carbonaceous structure (soft carbon), or a combinationthereof. Moreover, while a case where a so-called flake graphite is usedas the negative electrode active material 710 has been illustrated, thenegative electrode active material 710 is not limited to the illustratedexample.

<Thickener, Solvent>

The negative electrode active material layer 243 is formed by, forexample, preparing a negative electrode mixture in which the negativeelectrode active material 710 and the binder 730 described above aremixed into a solvent to a paste form (slurry), coating the negativeelectrode current collector 241 with the negative electrode mixture, andsubsequently performing drying and rolling. In doing so, any aqueoussolvent or non-aqueous solvent can be used for the solvent of thenegative electrode mixture. A preferable example of a non-aqueoussolvent is N-methyl-2-pyrrolidone (NMP). For the binder 730, polymermaterials exemplified as the binder 630 of the positive electrode activematerial layer 223 (refer to FIG. 4) described earlier can be used. Inaddition, the above-mentioned polymer materials exemplified as thebinder 630 of the positive electrode active material layer 223 can alsobe used for the purpose of demonstrating a function as a thickener oranother additive of the positive electrode mixture in addition tofunctioning as a binder.

<Separators 262 and 264>

As shown in FIGS. 1 and 2, the separators 262 and 264 are members thatseparate the positive electrode sheet 220 and the negative electrodesheet 240 from each other. In this example, the separators 262 and 264are constituted by band-like sheet members with a predetermined widthwhich include a plurality of minute holes. For example, a separatorwhich is made of a porous polyolefin-based resin and which has asingle-layer structure or a laminated structure can be used as theseparators 262 and 264. In this example, as shown in FIGS. 2 and 3, awidth b1 of the negative electrode active material layer 243 is slightlywider than a width a1 of the positive electrode active material layer223. Furthermore, widths c1 and c2 of the separators 262 and 264 areslightly wider than the width b1 of the negative electrode activematerial layer 243 (c1, c2>b1>a1).

Moreover, in the example shown in FIGS. 1 and 2, the separators 262 and264 are constituted by sheet-shaped members. The separators 262 and 264may be a member which insulates the positive electrode active materiallayer 223 and the negative electrode active material layer 243 from eachother and which allows transfer of an electrolyte. Therefore, theseparators 262 and 264 are not limited to sheet-shaped members. Insteadof sheet-shaped members, the separators 262 and 264 may be constitutedby, for example, a layer of particles which have insulating propertiesand which are formed on a surface of the positive electrode activematerial layer 223 or the negative electrode active material layer 243.In this case, the particles having insulating properties may beconstituted by an inorganic filler (for example, a filler such as ametal oxide or a metal hydroxide) having insulating properties or resinparticles (for example, particles of polyethylene or polypropylene)having insulating properties.

<Battery Case 300>

Furthermore, in this example, as shown in FIG. 1, the battery case 300is a so-called square battery case and comprises a container main body320 and a lid 340. The container main body 320 has a bottomed squaretube shape and is a flat box-shaped container with one side surface(upper surface) opened. The lid 340 is a member which is attached to theopening (upper surface opening) of the container main body 320 and whichblocks the opening.

With a vehicle-mounted secondary battery, weight energy efficiency(capacity of battery per unit weight) is desirably improved in order toimprove fuel efficiency of the vehicle. Therefore, in the presentembodiment, a light-weight metal such as aluminum or an aluminum alloyis adopted as the container main body 320 and the lid 340 constitutingthe battery case 300. Accordingly, weight energy efficiency can beimproved.

The battery case 300 has a flat rectangular inner space as a space forhousing the wound electrode body 200. In addition, as shown in FIG. 1, awidth of the flat inner space of the battery case 300 is slightlygreater than the wound electrode body 200. In the present embodiment,the battery case 300 comprises the container main body 320 having abottomed square tube shape and the lid 340 that blocks the opening ofthe container main body 320. Furthermore, electrode terminals 420 and440 are attached to the lid 340 of the battery case 300. The electrodeterminals 420 and 440 penetrate the battery case 300 (the lid 340) andreach the outside of the battery case 300. Moreover, an inlet 350 and asafety valve 360 are provided on the lid 340.

As shown in FIG. 2, the wound electrode body 200 is flatly deformed inone direction that is perpendicular to a winding axis WL. In the exampleshown in FIG. 2, on both sides of the separators 262 and 264, theuncoated portion 222 of the positive electrode current collector 221 andthe uncoated portion 242 of the negative electrode current collector 241are respectively spirally exposed. As shown in FIG. 6, in the presentembodiment, intermediate portions 224 and 244 of the uncoated portions222 and 242 are assembled and respectively welded to tips 420 a and 440a of the electrode terminals 420 and 440. When doing so, due todifferences in the respective materials, for example, ultrasonic weldingis used to weld the electrode terminal 420 and the positive electrodecurrent collector 221 to each other. In addition, for example,resistance welding is used to weld the electrode terminal 440 and thenegative electrode current collector 241 to each other. Here, FIG. 6 isa side view showing a welding location of the intermediate portion 224(244) of the uncoated portion 222 (242) and the electrode terminal 420(440) of the wound electrode body 200 and is a sectional view takenalong VI-VI in FIG. 1.

The wound electrode body 200 is attached to the electrode terminals 420and 440 fixed to the lid 340 in a state where the wound electrode body200 is pressed and bent flat. As shown in FIG. 1, this wound electrodebody 200 is housed in the flat inner space of the container main body320. After the wound electrode body 200 is housed, the container mainbody 320 is blocked by the lid 340. A joint 322 (refer to FIG. 1) of thelid 340 and the container main body 320 is welded and sealed by, forexample, laser welding. As described above, in this example, the woundelectrode body 200 is positioned inside the battery case 300 by theelectrode terminals 420 and 440 fixed to the lid 340 (the battery case300).

Electrolyte Solution>

Subsequently, an electrolyte solution is injected into the battery case300 from the inlet 350 provided on the lid 340. A so-called non-aqueouselectrolyte solution which does not use water as a solvent is used asthe electrolyte solution. In this example, as the electrolyte solution,an electrolyte solution in which LiPF₆ is contained at a concentrationof approximately 1 mol/liter in a mixed solvent of ethylene carbonateand diethyl carbonate (for example, a mixed solvent with a volume ratioof around 1:1) is used. Subsequently, a metallic sealing cap 352 isattached (for example, welded) to the inlet 350 to seal the battery case300. Moreover, the electrolyte solution is not limited to theelectrolyte solution exemplified herein. For example, non-aqueouselectrolyte solutions conventionally used in lithium-ion secondarybatteries may be used as appropriate.

<Holes>

The positive electrode active material layer 223 has minute gaps 225which may be described as cavities between, for example, the positiveelectrode active material particles 610 and particles of theelectrically conductive material 620 (refer to FIG. 4). An electrolytesolution (not shown) penetrates into the minute gaps of this positiveelectrode active material layer 223. In addition, the negative electrodeactive material layer 243 has minute gaps 245 which may be described ascavities between, for example, particles of the negative electrodeactive material 710 (refer to FIG. 5). Here, these gaps 225 and 245(cavities) will be referred to as “holes” when appropriate. In addition,with the wound electrode body 200, the uncoated portions 222 and 242 arespirally wound on both sides along the winding axis WL as shown in FIG.2. An electrolyte solution may penetrate from gaps of the uncoatedportions 222 and 242 on both sides 252 and 254 along this winding axisWL. As a result, the electrolyte solution penetrates into the positiveelectrode active material layer 223 and the negative electrode activematerial layer 243 inside the lithium-ion secondary battery 100.

<Outgassing Path>

In addition, in this example, the flat inner space of the battery case300 is slightly wider than the flatly-deformed wound electrode body 200.Gaps 310 and 312 are provided on both sides of the wound electrode body200 between the wound electrode body 200 and the battery case 300. Thegaps 310 and 312 act as outgassing paths.

In this lithium-ion secondary battery 100, the positive electrodecurrent collector 221 and the negative electrode current collector 241are electrically connected to an external device through the electrodeterminals 420 and 440 which penetrate the battery case 300. Operationsof the lithium-ion secondary battery 100 during charging and dischargingwill now be described.

<Operation During Charging>

FIG. 7 schematically shows a state of this lithium-ion secondary battery100 during charging. During charging, as shown in FIG. 7, the electrodeterminals 420 and 440 (refer to FIG. 1) of the lithium-ion secondarybattery 100 are connected to a charger 290. Due to the effect of thecharger 290, during charging, lithium ions (Li) are released from thepositive electrode active material in the positive electrode activematerial layer 223 into the electrolyte solution 280. At the same time,an electric charge is released from the positive electrode activematerial layer 223. The released electric charge is sent to the positiveelectrode current collector 221 via the electrically conductive material(not shown), and are further sent to the negative electrode 240 via thecharger 290. Meanwhile, at the negative electrode 240, an electriccharge is stored and, at the same time, the lithium ions (Li) in theelectrolyte solution 280 are adsorbed and stored by the negativeelectrode active material in the negative electrode active materiallayer 243.

<Operation During Discharging>

FIG. 8 schematically shows a state of this lithium-ion secondary battery100 during discharging. During discharging, as shown in FIG. 8, anelectric charge is sent from the negative electrode sheet 240 to thepositive electrode sheet 220 and, at the same time, lithium ions storedin the negative electrode active material layer 243 are released intothe electrolyte solution 280. In addition, at the positive electrode,the lithium ions in the electrolyte solution 280 are absorbed by thepositive electrode active material in the positive electrode activematerial layer 223.

In this manner, during charging and discharging of the lithium-ionsecondary battery 100, lithium ions migrate between the positiveelectrode active material layer 223 and the negative electrode activematerial layer 243 via the electrolyte solution 280. In addition, duringcharging, an electric charge is sent from the positive electrode activematerial to the positive electrode current collector 221 via theelectrically conductive material. In contrast, during discharging, anelectric charge is returned from the positive electrode currentcollector 221 to the positive electrode active material via theelectrically conductive material.

During charging, conceivably, the smoother the migration of the lithiumions and the transfer of electrons, the higher the efficiency and thespeed of charging that can be performed. During discharging,conceivably, the smoother the migration of the lithium ions and thetransfer of electrons, the lower the resistance of the battery and thegreater the discharge capacity, which results in improved batteryoutput.

<Other Battery Modes>

Moreover, the above description represents an example of a lithium-ionsecondary battery. However, lithium-ion secondary batteries are notlimited to the mode described above. Similarly, an electrode sheetobtained by coating a metallic foil with an electrode mixture may beused in various other battery modes. For example, a cylindrical batteryand a laminated battery are known as other battery modes. A cylindricalbattery is a battery in which a wound electrode body is housed in acylindrical battery case. In addition, a laminated battery is a batteryin which a positive electrode sheet and a negative electrode sheet arelaminated with a separator interposed between the positive electrodesheet and the negative electrode sheet Moreover, while the lithium-ionsecondary battery 100 is exemplified above, secondary batteries otherthan a lithium-ion secondary battery may also adopt similar structures.

Hereinafter, a lithium-ion secondary battery according to an embodimentof the present invention will be described. Moreover, since thelithium-ion secondary battery described below has a same basic structureas the lithium-ion secondary battery 100 described above, reference willbe made to the drawings of the lithium-ion secondary battery 100described above as appropriate for the following description.

As shown in FIG. 1, the lithium-ion secondary battery 100 comprises thepositive electrode current collector 221 and the porous positiveelectrode active material layer 223. As shown in FIG. 5, the positiveelectrode active material layer 223 is retained by the positiveelectrode current collector 221 and contains the positive electrodeactive material particles 610 (positive electrode active material), theelectrically conductive material 620, and the binder 630.

<Positive Electrode Active Material Particles 610>

As shown in FIG. 9, the positive electrode active material particles 610comprise a shell portion 612 constituted by a lithium transition metaloxide, a hollow portion 614 formed inside the shell portion 612, and athrough hole 616 penetrating the shell portion 612. Moreover, in thiscase, a portion corresponding to the through hole 616 of the positiveelectrode active material particles 610 among an inner surface 612 a ofthe shell portion 612 is not included in the inner surface 612 a of theshell portion 612. In addition, the through hole 616 is not included inthe hollow portion 614 of the positive electrode active materialparticles 610.

In the lithium-ion secondary battery 100, in the positive electrodeactive material layer 223 on average, a ratio of the hollow portion 614among an apparent sectional area of the positive electrode activematerial particles 610 is 23% or higher. Furthermore, on an arbitrarycross section of the positive electrode active material layer 223, athickness T(k) of the shell portion 612 at an arbitrary position k onthe inner surface of the shell portion 612 is assumed to be a shortestdistance T(k) from the arbitrary position k to an outer surface of theshell portion 612. In this case, in the lithium-ion secondary battery100, a thickness T of the shell portion 612 on an arbitrary crosssection of the positive electrode active material layer 223 on averageis 2.2 μm or less. Here, an “apparent sectional area of the positiveelectrode active material particles 610” is a sectional area of thepositive electrode active material particles 610 including the hollowportion.

<Ratio of Hollow Portion 614:Particle Porosity>

The ratio of the hollow portion 614 among the apparent sectional area ofthe positive electrode active material particles 610 can be ascertainedbased on sectional SEM images of the positive electrode active materiallayer 223. With a sectional SEM image of the positive electrode activematerial layer 223, as shown in FIG. 9, the shell portion 612, thehollow portion 614, and the through hole 616 of the positive electrodeactive material particles 610 can be distinguished based on differencesin tonality and grayscale among the sectional SEM image of the positiveelectrode active material layer 223.

In addition, based on an arbitrary sectional SEM image of the positiveelectrode active material layer 223, a ratio (A/B) of an area A occupiedby the hollow portion 614 of the positive electrode active materialparticles 610 among the sectional SEM image and a sectional area Bapparently occupied by the positive electrode active material particles610 is obtained. Here, the sectional area B apparently occupied by thepositive electrode active material particles 610 is a sectional areaoccupied by the shell portion 612, the hollow portion 614, and thethrough hole 616 of the positive electrode active material particles610.

Furthermore, an average value of the ratio (A/B) described above isobtained from a plurality of arbitrary sectional SEM images of thepositive electrode active material layer 223. The larger the number ofthe sectional SEM images from which the ratio (A/B) of the area among asectional SEM image is obtained, the stronger the convergence of theaverage value of the ratio (A/B) with respect to the positive electrodeactive material layer 223. Based on the average value of this ratio(A/B), a ratio of the hollow portion 614 among the apparent sectionalarea of the positive electrode active material particles 610 in thepositive electrode active material layer 223 on average can beapproximately obtained. The ratio of the hollow portion 614 among theapparent sectional area of the positive electrode active materialparticles 610 in the positive electrode active material layer 223 onaverage will be referred to as a “particle porosity” as appropriate.

<Thickness T of Shell Portion 612>

In this case, the shortest distance T(k) described above is obtained ata plurality of positions on the inner surface 612 a of the shell portion612. Subsequently, an average of the shortest distances T(k) obtained ata plurality of positions on the inner surface 612 a of the shell portion612 may be calculated. In this case, the larger the number of positionson the inner surface 612 a of the shell portion 612 at which theshortest distance T(k) described above is obtained, the stronger theconvergence of the thickness T of the shell portion 612 to an averagevalue, which enables the thickness of the shell portion 612 to bereflected. If the shell portion 612 has a distorted sectional shape, itis difficult to unambiguously define a thickness. With this method,since the thickness of the shell portion 612 is unambiguously determinedat an arbitrary position k on the inner surface 612 a of the shellportion 612, the thickness T of the shell portion 612 over the entirepositive electrode active material particles 610 can be approximatelyunambiguously defined.

As shown in FIG. 9, these positive electrode active material particles610 have the shell portion 612, the hollow portion 614, and the throughhole 616, and an inside of the shell portion 612 (the hollow portion614) and the outside of the shell portion 612 communicate with eachother through the through hole 616. These positive electrode activematerial particles 610 will be referred to as a holed hollow structureas appropriate. The hollow portion 614 of these positive electrodeactive material particles 610 is spacious and, for example, the ratio ofthe hollow portion 614 among the apparent sectional area of the positiveelectrode active material particles 610 may be 23% or higher. Inaddition, these positive electrode active material particles 610 havethe through hole 616 that penetrates the shell portion 612. Therefore,the electrolyte solution 280 (refer to FIGS. 7 and 8) also penetrates tothe inside of shell portion 612 (the hollow portion 614) via the throughhole 616. In the positive electrode active material particles 610, thehollow portion 614 is spacious. Therefore, the electrolyte solution 280containing lithium ions sufficiently exists not only outside the shellportion 612 but also inside the shell portion 612 (the hollow portion614). Furthermore, in the lithium-ion secondary battery 100, the shellportion 612 of the positive electrode active material particles 610 onan arbitrary cross section of the positive electrode active materiallayer 223 on average is thin and has a thickness T of 2.2 μm or less.

According to findings made by the present inventors, the thinner thethickness T of the shell portion 612 of the positive electrode activematerial particles 610, the more readily the lithium ions are releasedfrom the inside of the shell portion 612 of the positive electrodeactive material particles 610 during charging and the more readily thelithium ions are absorbed even to the inside of the shell portion 612 ofthe positive electrode active material particles 610 during discharging.

In this lithium-ion secondary battery 100, in the positive electrodeactive material layer 223 on average, the ratio of the hollow portion614 among the apparent sectional area of the positive electrode activematerial particles 610 is 23% or higher, the positive electrode activematerial particles 610 have a through hole 616 penetrating the shellportion 612, and the thickness T of the shell portion 612 of thepositive electrode active material particles 610 is extremely thin (inthis case, 2.2 μm or less). Therefore, lithium ions diffuse rapidly intothe shell portion 612 (inside of the active material) of the positiveelectrode active material particles 610. In other words, lithium ionsare readily released even from the inside of the shell portion 612 ofthe positive electrode active material particles 610 during charging,and the lithium ions are readily absorbed even to the inside of theshell portion 612 of the positive electrode active material particles610 during discharging.

As shown, during charging and discharging of the lithium-ion secondarybattery 100, the positive electrode active material particles 610contribute toward smooth release and absorption of the lithium ions evento the inside of the shell portion 612. Consequently, amounts of releaseand absorption of the lithium ions per unit weight of the positiveelectrode active material particles 610 can be increased, and aresistance during the release and absorption of the lithium ions by thepositive electrode active material particles 610 can be reduced.Therefore, output of the lithium-ion secondary battery 100 is lesslikely to decrease ever when the amount of charge is low. In otherwords, with the lithium-ion secondary battery 100, lithium ions arereadily released even from the inside of the shell portion 612 of thepositive electrode active material particles 610, and the lithium ionsare readily absorbed even to the inside of the shell portion 612, of thepositive electrode active material particles 610. Therefore, thelithium-ion secondary battery 100 is capable of producing a requiredoutput even if the amount of charge is low.

As described above, the positive electrode active material particles 610have the shell portion 612, the hollow portion 614, and the through hole616. In addition, the hollow portion 614 is spacious and the shellportion 612 is thin. Previously, such positive electrode active materialparticles 610 are generally unknown. For example, the ratio of thehollow portion 614 among an apparent sectional area of the positiveelectrode active material particles 610 is 23% or higher and, therefore,may be distinctly distinguished from a simple sintered body.

<Production Method of Positive Electrode Active Material Particles 610>

Hereinafter, a preferable production method of these positive electrodeactive material particles 610 which is capable of stably producing thepositive electrode active material particles 610 will be described.

The production method of the positive electrode active materialparticles 610 comprises, for example, a raw material hydroxide formationstep, a mixing step, and a calcining step. The raw material hydroxideformation step is a step of supplying ammonium ions to an aqueoussolution of a transition metal compound and precipitating particles ofthe transition metal hydroxide from the aqueous solution. In this case,the aqueous solution contains at least one transition metal element thatcomposes the lithium transition metal oxide.

The mixing step is a step of mixing the transition metal hydroxide witha lithium compound to prepare an unfired mixture. The calcining step isa step of calcining the mixture to obtain the positive electrode activematerial particles 610. Furthermore, preferably, the fired product maybe crushed and sieved after the calcining.

In this case, the raw material hydroxide formation step may include anucleation stage in which the transition metal hydroxide is precipitatedfrom the aqueous solution and a particle growth stage in which thetransition metal hydroxide is grown in a state where a pH of the aqueoussolution is lowered from the nucleation stage.

Hereinafter, the production method of the positive electrode activematerial particles 610 will be exemplified more specifically.

The holed hollow active material particles disclosed herein can beproduced by, for example, precipitating a hydroxide of a transitionmetal under suitable conditions from an aqueous solution containing atleast one transition metal element contained in a lithium transitionmetal oxide that composes the active material particles (and favorably,all transition metals other than lithium contained in the oxide), andmixing the transition metal hydroxide with a lithium compound followedby calcining. Although the following provides a detailed explanation ofan embodiment of this active material particle production method byusing, as an example, a case of producing holed hollow active materialparticles composed of a LiNiCoMn oxide and having a layered structure,this explanation is not intended to limit the application target of thisproduction method to holed hollow active material particles composed inthis manner.

The active material particle production method disclosed herein includesa step of supplying ammonium ions (NH₄ ⁺) to an aqueous solution of atransition metal compound and precipitating particles of transitionmetal hydroxide from the aqueous solution (raw material hydroxideformation step). The solvent (aqueous solvent) that composes the aqueoussolution is typically water, and may also be a mixed solvent composedmainly of water. An organic solvent able to uniformly mix with water(such as a lower alcohol) is preferable as a solvent other than waterthat composes the mixed solvent. The aqueous solution of the transitionmetal compound (to also be referred to hereinafter as a “transitionmetal solution”) contains at least one (favorably all) transition metalelement(s) (here, Ni, Co and Mn) that composes the lithium transitionmetal oxide corresponding to the composition of the lithium transitionmetal oxide that composes the active material particles targeted forproduction. For example, a transition metal solution is used thatcontains one type or two or more types of compounds that are capable ofsupplying Ni ions, Co ions and Mn ions to the aqueous solvent. Examplesof compounds serving as the source of these metal ions that can be usedpreferably include sulfates, nitrates and chlorides of the metals. Forexample, a transition metal solution can be used favorably that has acomposition in which nickel sulfate, cobalt sulfate and manganesesulfate are dissolved in an aqueous solvent (favorably water).

The above-mentioned NH₄ ⁺ ions may be supplied to the transition metalsolution in the form of an aqueous solution containing NH₄ ⁺ ions(typically a water-based solution), may be supplied to the transitionmetal solution by directly blowing in ammonia gas, or may be supplied bya combination thereof. An aqueous solution containing NH₄ ⁺ ions can beprepared by, for example, dissolving a compound capable of serving as anNH₄ ⁺ ion source (such as ammonium hydroxide, ammonium nitrate orammonia gas) in an aqueous solvent. In the present embodiment, NH₄ ⁺ions are supplied in the form of an aqueous ammonium hydroxide solution(namely, ammonia water).

The above-mentioned raw material hydroxide formation step can include astage in which a transition metal hydroxide is precipitated from thetransition metal solution under conditions of a pH of 12 or higher (andtypically, pH 12 to pH 14, and for example, pH 12.2 to pH 13) and at anNH₄ ⁺ concentration of 25 g/L or lower (and typically, 3 to 25 g/L)(nucleation stage). The pH and the NH₄ ⁺ concentration can be adjustedby suitably balancing the amounts of the ammonia water and an alkalineagent (a compound having an action that causes a liquid to becomealkaline) used. Sodium hydroxide or potassium hydroxide, for example,can be used for the alkaline agent, typically in the form of an aqueoussolution. In the present embodiment, an aqueous sodium hydroxidesolution is used. In the present specification, pH values refer to pHvalues based on a liquid temperature of 25° C.

The above-mentioned raw material hydroxide formation step can furtherinclude a stage in which cores (typically, particulate) of thetransition metal hydroxide precipitated in the above-mentionednucleation stage are grown at a pH lower than 12 (typically, pH 10 orhigher and lower than pH 12, favorably pH 10 to pH 11.8, and forexample, pH 11 to pH 11.8) and an NH₄ ⁺ concentration of 1 g/L orhigher, and favorably 3 g/L or higher (typically, 3 to 25 g/L) (particlegrowth stage). Normally, the pH of the particle growth stage is 0.1 orhigher (typically 0.3 or higher, favorably 0.5 or higher, and forexample, about 0.5 to 1.5) lower than the pH of the nucleation stage.

The pH and the NH₄ ⁺ concentration can be adjusted in the same manner asin the nucleation stage. By carrying out this particle growth stage soas to satisfy the above-mentioned pH and NH₄ ⁺ concentration, and makingthe NH₄ ⁺ concentration at the above-mentioned pH to favorably be withinthe range of 15 g/L or lower (for example, 1 to 15 g/L and typically 3to 15 g/L) and more favorably within the range of 10 g/L or lower (forexample, 1 to 10 g/L, and typically 3 to 10 g/L), the precipitation rateof the transition metal hydroxide (here, a complex hydroxide containingNi, Co and Mn) increases, and raw material hydroxide particles can beformed that are suitable for forming the holed hollow active materialparticles disclosed herein (or in other words, raw material hydroxideparticles that easily form a fired product having a holed hollowstructure).

The above-mentioned NH₄ ⁺ concentration may also be made to be 7 g/L orlower (for example, 1 to 7 g/L and more favorably 3 to 7 g/L). The NH₄ ⁺concentration in the particle growth stage may be, for example, roughlyequal to the NH₄ ⁺ concentration in the nucleation stage or may be lowerthan the NH₄ ⁺ concentration in the nucleation stage. The precipitationrate of the transition metal hydroxide can be determined by, forexample, investigating the change in the total number of moles oftransition metal ions contained in the liquid phase of the reactionsolution (total ion concentration) relative to the total number of molesof transition metal ions contained in the transition metal solutionsupplied to the reaction solution.

The temperature of the reaction solution in each of the nucleation stageand particle growth stage is favorably controlled to a nearly constanttemperature (for example, a prescribed temperature ±1° C.) within arange of roughly 30° C. to 60° C. The temperatures of the reactionsolutions in the nucleation stage and the particle growth stage may bethe same. In addition, the atmosphere in the reaction solutions and thereaction tanks is favorably maintained at a non-oxidizing atmospherethroughout the nucleation stage and the particle growth stage. Inaddition, the total number of moles of Ni ions, Co ions and Mn ionscontained in the reaction solution (total ion concentration) is made tobe, for example, roughly 0.5 to 2.5 mol/L, and favorably about 1.0 to2.2 mol/L, throughout the nucleation stage and the particle growthstage. The transition metal solution may be replenished (typically,supplied continuously) according to the precipitation rate of thetransition metal hydroxide so as to maintain this total ionconcentration. The amounts of Ni ions, Co ions and Mn ions contained inthe reaction solution are favorably set to a quantity ratio thatcorresponds to the composition of the target active material particles(namely, the molar ratio of Ni, Co and Mn in the LiNiCoMn oxide thatcomposes the active material particles).

In the present embodiment, the transition metal hydroxide particles(here, complex hydroxide particles containing Ni, Co and Mn) formed inthe manner described above are separated from the reaction solution, andwashed and dried. An unfired mixture is then prepared by mixing thetransition metal hydroxide particles and a lithium compound at a desiredquantity ratio (mixing step). In this mixing step, the Li compound andthe transition metal hydroxide particles are typically mixed at aquantity ratio corresponding to the composition of the target activematerial particles (namely, the molar ratio of Li, Ni, Co and Mn in theLiNiCoMn oxide that composes the active material particles). Examples ofthe lithium compound that can be used favorably include Li compoundssuch as lithium carbonate or lithium hydroxide that can become oxides asa result of melting with heat.

The above-mentioned mixture is then fired to obtain active materialparticles (calcining step). This calcining step is typically carried outin an oxidizing atmosphere (for example, in the air). The calciningtemperature in this calcining step can be, for example, 700° C. to 1100°C. The calcining step is favorably carried out so that a maximumcalcining temperature is 800° C. or higher (favorably 800° C. to 1100°C. and for example, 800° C. to 1050° C.). As a result of the maximumcalcining temperature being within these ranges, a sintering reaction ofprimary particles of a lithium transition metal oxide (favorably anNi-containing Li oxide and, here, an LiNiCoMn oxide) can be allowed toproceed suitably.

In a favorable aspect thereof, the calcining step is carried out in anaspect that includes a first calcining stage, in which the mixture isfired at a temperature T1 of 700° C. to 900° C. (namely, 700° C.≦T1≦900°C., for example, 700° C.≦T1≦800° C., and typically 700° C.≦T1<800° C.),and a second calcining stage, in which the result of the first calciningstage is fired at a temperature T2 of 800° C. to 1100° C. (namely, 800°C.≦T2≦1100° C., and for example, 800° C.≦T2≦1050° C.). As a result,active material particles having a holed hollow structure can be formedmore efficiently. T1 and T2 are favorably set such that T1<T2.

The first calcining stage and the second calcining stage may be carriedout continuously (by, for example, holding the mixture at the firstcalcining temperature T1 following raising the temperature of themixture to the second calcining temperature T2 and holding at thatcalcining temperature T2), or after having held at the first calciningtemperature T1, the mixture may be temporarily cooled (by, for example,cooling to room temperature) and then supplying the mixture to thesecond calcining stage after having crushed and sieved the mixture asnecessary.

In the technology disclosed herein, the first calcining stage can beunderstood to be a stage during which calcining is carried out at thetemperature T1 that is within a temperature range at which the sinteringreaction of the target lithium transition metal oxide progresses, isequal to or lower than the melting point thereof, and is lower than thatof the second calcining stage. In addition, the second calcining stagecan be understood to be a stage at which calcining is carried out at atemperature T2 that is within a temperature range at which the sinteringreaction of the target lithium transition metal oxide progresses, isequal to or lower than the melting point thereof, and is higher thanthat of the first calcining stage. A temperature difference of 50° C. orhigher (typically 100° C. or higher, and for example, 150° C. or higher)is favorably provided between T1 and T2.

As described above, the production method of the positive electrodeactive material particles 610 comprises the raw material hydroxideformation step, the mixing step, and the calcining step. In this case,the positive electrode active material particles 610 may be stablyobtained such that the ratio of the hollow portion 614 among theapparent sectional area of the positive electrode active materialparticles 610 is 23% or higher and the shell portion 612 of the positiveelectrode active material particles 610 is thin with a thickness T of2.2 μm or less. Hereinafter, a production method of the positiveelectrode active material particles 610 which is capable of producingthese positive electrode active material particles 610 in a more stablemanner will be described.

<Raw Material Hydroxide Formation Step>

In order to obtain the positive electrode active material particles 610more stably, for example, the pH or the NH4+ concentration of the stagein which a transition metal hydroxide is precipitated from a transitionmetal solution (nucleation stage) and the pH or the NH4+ concentrationof the stage in which cores of the transition metal hydroxideprecipitated in the nucleation stage are grown (particle growth stage)are appropriately adjusted.

In this transition metal solution, for example, equilibrium reactionssuch as those presented below are taking place.(M1)²⁺+(NH₃)

[M1(NH₃)₆]²⁺  Formula 1, and(M1)²⁺+2OH⁻

M1(OH)₂  Formula 2,where, M1 represents transition metals contained in the transition metalsolution and, in the present embodiment, includes Ni.

In other words, in the equilibrium reaction represented by Formula 1, atransition metal (M1) in the transition metal solution, ammonia (NH₃)supplied to the transition metal solution, and a compound ([M1(NH₃)₆]²⁺)of the transition metal (M1) and the ammonia (NH₃) are in equilibrium.In the equilibrium reaction represented by Formula 2, the transitionmetal (M1) in the transition metal solution, hydroxide ions (OH⁻)supplied to the transition metal solution, and a transition metalhydroxide (M1(OH)₂) are in equilibrium.

In this case, when the pH in the transition metal solution decreases,the transition metal hydroxide (M1(OH)₂) is more readily precipitateddue to the equilibrium reaction represented by Formula 2. At this point,by keeping the amount of ammonia in the transition metal solution low sothat the equilibrium equation represented by Formula 1 proceeds towardthe left-hand side and the transition metal ions (M1)²⁺ in thetransition metal solution increases, the transition metal hydroxide(M1(OH)₂) is more readily precipitated. In this manner, by keeping theamount of ammonia in the transition metal solution low and, at the sametime, lowering the pH in the transition metal solution, the transitionmetal hydroxide (M1(OH)₂) is more readily precipitated.

For example, in the nucleation stage, solubility of ammonia (NH₃) in thetransition metal solution is kept low while the pH is kept relativelyhigh. Accordingly, a precipitation rate of the transition metalhydroxide (M1(OH)₂) can be adequately suppressed. As a result, a densityinside particles of the transition metal hydroxide which become aprecursor can be lowered. Furthermore, in the particle growth stage,solubility of ammonia (NH₃) in the transition metal solution is kept lowwhile lowering the pH. Accordingly, the precipitation rate of thetransition metal hydroxide (M1(OH)₂) increases in the nucleation stage.As a result, the density in a vicinity of outer surfaces of theparticles of the transition metal hydroxide which becomes a precursorbecomes higher than the density inside the particles of the transitionmetal hydroxide.

As described above, by appropriately adjusting the pH and the ammoniaconcentration (ammonium ion concentration) of the transition metalsolution in the nucleation stage and the particle growth stage, thedensity of the transition metal hydroxide inside the particles can belowered and the density of the transition metal hydroxide in thevicinity of the outer surfaces can be increased.

In this case, for example, the pH of the transition metal solution inthe nucleation stage may be 12 to 13, and the pH of the aqueous solutionin the particle growth stage may be 11 or higher and lower than 12. Atthis point, favorably, the pH of the transition metal solution in thenucleation stage has been lowered from the particle growth stage by 0.1or more and favorably by 0.2 or more. In addition, the ammoniaconcentration (ammonium ion concentration) in the particle growth stagemay be kept low at 3 g/L to 10 g/L. Accordingly, the precipitation rateof the transition metal hydroxide (M1(OH)₂) can reliably be set higherin the particle growth stage than in the nucleation stage. As a result,the density in a vicinity of outer surfaces of the particles of thetransition metal hydroxide becomes reliably higher than the densityinside the particles of the transition metal hydroxide.

Moreover, by securing a required amount of time for the nucleationstage, the hollow portion 614 of the positive electrode active materialparticles 610 can be enlarged. In addition, by increasing theprecipitation rate of the transition metal hydroxide in the particlegrowth stage and reducing a duration of the particle growth stage, theshell portion 612 of the positive electrode active material particles610 can be made thinner.

Furthermore, in this case, the amount of ammonia in the transition metalsolution may be kept low. For example, an ammonium ion concentration inthe transition metal solution in the nucleation stage may be 20 g/L orlower, and the ammonium ion concentration in the transition metalsolution in the particle growth stage may be 10 g/L or lower. Asdescribed above, by keeping the ammonium ion concentration of thetransition metal solution in the nucleation stage and the particlegrowth stage at a low level, the concentration of transition metal ionscontained in the transition metal solution can be maintained at arequired level. In this case, an excessively small amount of ammonia inthe transition metal solution is also unfavorable. The ammonium ionconcentration of the transition metal solution in the nucleation stageand the particle growth stage may be, for example, 3 g/L or higher.

<Mixing Step, Calcining Step>

In the mixing step, the transition metal hydroxide and a lithiumcompound are mixed to prepare an unfired mixture. In the calcining step,the mixture is calcined to obtain the positive electrode active materialparticles 610. In this case, with the particles of the transition metalhydroxide which is a precursor of the positive electrode active materialparticles 610, the density inside is low and the density in the vicinityof the outer surface is high. Therefore, in the calcining step,sintering is performed so that the interior with the lower density amongthe particles of the transition metal hydroxide that is a precursor isincorporated into the vicinity of the outer surface having a higherdensity and a high mechanical strength. As a result, the shell portion612 as well as a spacious hollow portion 614 of the positive electrodeactive material particles 610 are formed. In addition, as crystals growduring sintering, the through hole 616 that penetrates the shell portion612 is formed in a part of the shell portion 612. Accordingly, as shownin FIG. 9, the positive electrode active material particles 610 havingthe shell portion 612, the hollow portion 614, and the through hole 616are formed. Furthermore, preferably, the fired product is crushed andsieved after the calcining step to adjust particle diameters of thepositive electrode active material particles 610.

The positive electrode active material particles 610 produced asdescribed above have a thin shell portion 612, a spacious hollow portion614, and a through hole 616 which penetrates the shell portion 612 andwhich spaciously connects the hollow portion 614 and the outside of theshell portion 612 of the positive electrode active material particles610 with each other. In a preferable mode of these positive electrodeactive material particles 610, a BET specific surface area of thepositive electrode active material particles 610 can be set toapproximately 0.3 m²/g to 2.2 m²/g. The BET specific surface area of thepositive electrode active material particles 610 may be more favorablyset to approximately 0.5 m²/g or more and even more favorably set toapproximately 0.8 m²/g or more. In addition, the BET specific surfacearea of the positive electrode active material particles 610 may be setto approximately 1.9 m²/g or less and more favorably set toapproximately 1.5 m²/g or less.

Furthermore, with these positive electrode active material particles610, as described above, the raw material hydroxide formation step isdivided into the nucleation stage and the particle growth stage and thedensity of the shell portion 612 is high. Therefore, positive electrodeactive material particles 610 are obtained which are harder and havehigh morphological stability than those produced by other methods (forexample, a spray firing method (also referred to as a spray dryingmethod)).

For example, these positive electrode active material particles 610 havean average hardness of 0.5 MPa or more as obtained by measuring dynamichardness under conditions of a loading speed of 0.5 mN/sec to 3 mN/secusing a flat diamond indenter having a diameter of 50 μm.

In addition, in another favorable aspect of the active materialparticles disclosed herein, the average hardness of the positiveelectrode active material particles 610 is approximately 0.5 MPa ormore. Here, average hardness refers to a value obtained by measuringdynamic microhardness under conditions of a loading speed of 0.5 mN/secto 3 mN/sec using a flat diamond indenter having a diameter of 50 μm.For example, a microhardness tester MCT-W500 manufactured by ShimadzuCorporation can be used for this dynamic microhardness measurement.

As described above, as shown in FIG. 9, the positive electrode activematerial particles 610 have a hollow structure and have a high averagehardness (in other words, high shape retention). These positiveelectrode active material particles 610 may provide a battery thatstably demonstrates higher performance. Therefore, the positiveelectrode active material particles 610 are extremely preferable forconstructing a lithium secondary battery that has low internalresistance (or in other words, favorable output characteristics) anddemonstrates little increase in internal resistance attributable tocharge-discharge cycling (particularly, charge-discharge cycling thatincludes high-rate discharge).

<Lithium Transition Metal Oxide Constituting Positive Electrode ActiveMaterial Particles 610>

In the production of these positive electrode active material particles610, particularly, the transition metal solution favorably containsnickel. When the transition metal solution contains nickel, as thetransition metal hydroxide is precipitated in the nucleation stage andthe particle growth stage, particles of the transition metal hydroxideare created in the form of secondary particles formed by an aggregationof a plurality of minute primary particles with shapes resembling ricegrains. In addition, in the temperature range during calcining, crystalsgrow while more or less maintaining the shapes of the primary particlesof this transition metal hydroxide.

On the other hand, in a case where the transition metal solutioncontains absolutely no nickel but contains cobalt instead and lithiumcobaltate (LiCoO₂) particles are created by calcining, the shape of theprimary particles cannot be maintained and entire particles end up beingsintered. As a result, positive electrode active material particles 610(refer to FIG. 9) having a spacious hollow portion 614 as describedabove may not be obtained.

As shown, in order to stably produce the positive electrode activematerial particles 610, the lithium transition metal oxide is preferablya compound which has a layered structure and which contains nickel as aconstituent element. By containing nickel in this manner, transitionmetal hydroxide particles (precursor particles) with low internaldensity and high density in the vicinity of the outer surface can beformed. In addition, based on the precursor particles with low internaldensity and high density in the vicinity of the outer surface, crystalscan be grown in the calcining step while more or less maintaining theshapes of the primary particles. Accordingly, the positive electrodeactive material particles 610 (refer to FIG. 9) having the shell portion612, the hollow portion 614, and the through hole 610 are formed.

In this case, the ratio (composition ratio) of nickel among thetransition metals contained in the positive electrode active materialparticles 610 may be approximately 0.1% or higher and, more favorably,0.25% or higher.

In addition, the lithium transition metal oxide may be a compound whichhas a layered structure and which contains nickel, cobalt, and manganeseas constituent elements. For example, the lithium transition metal oxidemay be a compound which has a layered structure and which is expressedas Li_(1+x)Ni_(y)Co_(z)Mn_((1-y-z))M_(γ), where 0≦x≦0.2, 0.1<y<0.9,0.1<z<0.4, M denotes an additive, and 0≦γ≦0.01. For example, M may be atleast one additive selected from the group consisting of Zr, W, Mg, Ca,Na, Fe, Cr, Zn, Si, Sn, Al, B, and F. This lithium transition metaloxide constitutes a compound with a layered structure and is capable ofretaining lithium ions between the layers. In addition, this lithiumtransition metal oxide is particularly preferable for producing thepositive electrode active material particles 610 described above whichhas the shell portion 612, the hollow portion 614, and the through hole616.

Accordingly, the positive electrode active material particles 610 can beobtained such that the ratio of the hollow portion 614 among theapparent sectional area of the positive electrode active materialparticles 610 is 23% or higher and the shell portion 612 of the positiveelectrode active material particles 610 is thin with a thickness T of2.2 μm or less.

As described earlier, as shown in FIGS. 1 to 3, the lithium-ionsecondary battery 100 comprises the positive electrode current collector221 (current collector) and the porous positive electrode activematerial layer 223 retained by the positive electrode current collector221. As shown in FIG. 4, this positive electrode active material layer223 contains the positive electrode active material particles 610, theelectrically conductive material 620, and the binder 630. In the presentembodiment, as shown in FIG. 9, the positive electrode active materialparticles 610 have the shell portion 612 constituted by a lithiumtransition metal oxide, the hollow portion 614 formed inside the shellportion 612, and the through hole 616 penetrating the shell portion 612.

As shown in FIG. 1, in the lithium-ion secondary battery 100, in thepositive electrode active material layer 223 on average, a ratio of thehollow portion 614 among an apparent sectional area of the positiveelectrode active material particles 610 (refer to FIG. 9) is 23% orhigher. In addition, on an arbitrary cross section of the positiveelectrode active material layer 223, a thickness of the shell portion612 at an arbitrary position on the inner surface 612 a of the shellportion 612 is assumed to be a shortest distance T(k) from the arbitraryposition k to an outer surface of the shell portion 612. In this case,the thickness of the shell portion 612 on an arbitrary cross section ofthe positive electrode active material layer 223 on average is 2.2 μm orless.

According to the lithium-ion secondary battery 100, as shown in FIG. 9,in the positive electrode active material layer 223 on average, a ratioof the hollow portion 614 among an apparent sectional area of thepositive electrode active material particles 610 (refer to FIG. 9) is23% or higher and the hollow portion 614 is spacious. In thislithium-ion secondary battery 100, the electrolyte solution 280 (referto FIGS. 7 and 8) sufficiently penetrates to the hollow portion 614 ofthe positive electrode active material particles 610 in the positiveelectrode active material layer 223. Furthermore, in the lithium-ionsecondary battery 100, the shell portion 612 on an arbitrary crosssection of the positive electrode active material layer 223 on averagehas a thickness of 2.2 μm or less and the shell portion 612 of thepositive electrode active material particles 610 is thin. Therefore,lithium ions diffuse rapidly to the inside of the shell portion 612(inside of the active material) of the positive electrode activematerial particles 610. As a result, the lithium-ion secondary battery100 can stably produce high output even when the amount of charge islow.

In this case, in the positive electrode active material layer 223 onaverage, the thickness of the shell portion 612 may be, for example,0.05 μm or more and, more favorably, 0.1 μm or more. When the thicknessof the shell portion 612 is 0.05 μm or more and, more favorably, 0.1 μmor more, the positive electrode active material particles 610 obtainnecessary mechanical strength. As lithium ions are repetitively releasedand absorbed, expansion and contraction occur in the positive electrodeactive material particles 610. Strength can be secured which is evensufficient with respect to such expansion and contraction. Therefore,durability of the positive electrode active material particles 610 isimproved and performance of the lithium-ion secondary battery 100 mayremain stable over time.

Furthermore, in the positive electrode active material layer 223 onaverage, an opening width of the through hole 616 may be on average 0.01μm or more. Here, the opening width of the through hole 616 refers tothe length across a portion where the through hole 616 is narrowestamong a path which reaches the hollow portion 614 from the outside ofthe positive electrode active material particles 610. When the openingwidth of the through hole 616 is on average 0.01 μm or more, theelectrolyte solution 280 (refer to FIG. 7 or 8) can sufficientlypenetrate into the hollow portion 614 through the through hole 616 fromthe outside. Accordingly, an effect of improving battery performance ofthe lithium-ion secondary battery 100 can be more appropriatelydemonstrated.

For example, normally, the thin shell portion 612, the spacious hollowportion 614, and the through hole 616 with the wide opening width suchas observed in the positive electrode active material particles 610cannot be realized by other production methods (for example, a sprayfiring method (also referred to as a spray drying method)).

An average value of the above-mentioned opening size (average openingsize) can be obtained by, for example, ascertaining opening sizes of aportion of or all of the through holes 616 possessed by at least tenpositive electrode active material particles 610, and then determiningthe arithmetic average thereof. In addition, the through hole 616 needonly be suitable for the penetration of the electrolyte solution 280 tothe hollow portion 614 and, in the positive electrode active materiallayer 223 on average, the opening width of the through hole 616 may beapproximately 2.0 μm or less.

<Evaluation of Positive Electrode Active Material Particles 610>

Hereinafter, the present inventors fabricated evaluation batteriesrespectively using positive electrode active material particles 610which differed from each other in terms of the ratio of the hollowportion 614 among an apparent sectional area, the thickness of the shellportion 612, and the presence or absence of the through hole 616 inorder to compare battery performances.

<Evaluation Battery>

A structure of the evaluation battery will now be described. Moreover,since the evaluation battery is a so-called flat, square battery such asthat shown in FIG. 1 and has a basic structure that is more or less thesame as the lithium-ion secondary battery 100 described earlier, thedescription will refer to the lithium-ion secondary battery 100 asappropriate. In addition, members or portions that produce same effectswill be denoted by same reference characters.

<Negative Electrode of Evaluation Battery 100>

As shown in FIGS. 1 and 5, the negative electrode of the evaluationbattery 100 comprises a negative electrode current collector 241 and anegative electrode active material layer 243 retained by the negativeelectrode current collector 241. The negative electrode active materiallayer 243 comprises negative electrode active material 710 and a binder730.

In the evaluation battery 100, a copper foil with a thickness ofapproximately 10 μm is used as the negative electrode current collector241. This negative electrode current collector 241 is a band-like sheetmember with a width of approximately 120 mm and a length ofapproximately 3200 mm, and an uncoated portion 242 in which the negativeelectrode active material layer 243 is not formed is set in a lengthwisedirection along one width-direction edge of the negative electrodecurrent collector 241. The negative electrode active material layer 243is retained on both surfaces of the negative electrode current collector241 in a portion (a portion with a width of approximately 105 mm)excluding the uncoated portion 242.

For the negative electrode active material 710 (refer to FIG. 5)contained in the negative electrode active material layer 243, 96% byweight of natural graphite powder was mixed and impregnated with 4% byweight of pitch. The mixture was then calcined for 10 hours at 1000° C.to 1300° C. under an inert atmosphere. The obtained negative electrodeactive material 710 was sieved so as to adjust an average particlediameter (median diameter D50) to approximately 8 to 11 μm and aspecific surface area to within a range of approximately 3.5 to 5.5m²/g.

The negative electrode active material layer 243 further contains athickener. The thickener is a material for adjusting viscosity of amixture prepared when forming the negative electrode active materiallayer 243. Here, carboxymethylcellulose (CMC) is used as this thickener.In addition, styrene-butadiene rubber (SBR) is used as the binder 730.

In this case, the negative electrode active material 710, the thickener,and the binder 730 are kneaded at a weight ratio of approximately98.6:0.7:0.7 together with water to prepare a paste-like negativeelectrode mixture (negative electrode paste). Subsequently, the negativeelectrode mixture is coated on both surfaces of the negative electrodecurrent collector 241 with the exception of the uncoated portion 242 sothat an amount of coating on each surface after drying is approximately7.5 mg/cm², and drying is performed to form the negative electrodeactive material layer 243. This negative electrode active material layer243 was further rolled by a roll pressing machine to obtain a density ofapproximately 1.0 to 1.4 g/cc. Accordingly, the negative electrode sheet240 (refer to FIG. 2) is obtained.

<Positive Electrode of Evaluation Battery 100>

As shown in FIGS. 1 and 6, the positive electrode of the evaluationbattery 100 comprises a positive electrode current collector 221 and apositive electrode active material layer 223 retained by the positiveelectrode current collector 221. The positive electrode active materiallayer 223 comprises positive electrode active material particles 610, anelectrically conductive material 620, and a binder 630 (refer to FIG.6).

In the evaluation battery 100, an aluminum foil with a thickness ofapproximately 15 μm is used as the positive electrode current collector221. This positive electrode current collector 221 is a band-like sheetmember with a width of approximately 115 mm and a length ofapproximately 3000 mm, and an uncoated portion 222 in which the positiveelectrode active material layer 223 is not formed is set in a lengthwisedirection along one width-direction edge of the positive electrodecurrent collector 221. The positive electrode active material layer 223is retained on both surfaces of the positive electrode current collector221 in a portion (a portion with a width of approximately 95 mm)excluding the uncoated portion 222.

<Positive Electrode Active Material Particles 610 of Evaluation Battery100>

For the positive electrode active material particles 610 (refer to FIG.4) contained in the positive electrode active material layer 223, amixed solution of nickel sulfate (NiSO₄), cobalt sulfate (CoSO₄), andmanganese sulfate (MnSO₄) is neutralized by sodium hydroxide (NaOH).Subsequently, a transition metal hydroxide that becomes a precursor isobtained in a process of supplying ammonium ions (NH4+) to thistransition metal compound aqueous solution and precipitating transitionmetal hydroxide particles from the aqueous solution (raw materialhydroxide formation step). To this end, in the evaluation battery, Ni,Co, and Mn are contained at approximately a predetermined ratio in thetransition metal hydroxide that becomes a precursor.

With the evaluation battery 100, in the mixing step described earlier,lithium carbonate (Li₂CO₃) is mixed into this transition metal hydroxidethat becomes a precursor. This mixture is then calcined for 10 hours at950° C. in the calcining step. Accordingly, positive electrode activematerial particles 610 with a basic composition ofLi_(1.15)Ni_(0.33)Co_(0.33)Mn_(0.33)O₂ was fabricated. As shown in FIG.9, the positive electrode active material particles 610 having the shellportion 612, the hollow portion 614, and the through hole 616 areformed. Furthermore, preferably, the fired product is crushed and sievedafter the calcining step to adjust particle diameters of the positiveelectrode active material particles 610.

Here, the raw material hydroxide formation step was adjusted to preparepositive electrode active material particles 610 which differed fromeach other in terms of the ratio of the hollow portion 614 among anapparent sectional area, the thickness of the shell portion 612, and thepresence or absence of the through hole 616. Moreover, in order tocomparatively evaluate how much the performance of the evaluationbattery 100 varies when the ratio of the hollow portion 614 among anapparent sectional area, the thickness of the shell portion 612, and thepresence or absence of the through hole 616 differ, the composition ofthe positive electrode active material particles 610 among respectivesamples was set approximately the same. Meanwhile, there is a certainamount of variation in the average particle diameter (D50) of thepositive electrode active material particles 610.

The positive electrode active material layer 223 contains the positiveelectrode active material particles 610, the electrically conductivematerial 620, and the binder 630. In the evaluation battery 100,acetylene black (AB) is used as the electrically conductive material 620and polyvinylidene fluoride (PVDF) is used as the binder 630 of thepositive electrode active material layer 223. The positive electrodeactive material particles 610, the electrically conductive material 620,and the binder 630 are kneaded at a weight ratio of 90:8:2 together withN-methyl-2-pyrrolidone (NMP) to prepare a paste-like positive electrodemixture (positive electrode paste).

Subsequently, the positive electrode mixture is coated on both surfacesof the positive electrode current collector 221 with the exception ofthe uncoated portion 222 so that an amount of coating on each surfaceafter drying is approximately 11.8 mg/cm², and drying is performed toform the positive electrode active material layer 223. This positiveelectrode active material layer 223 was further rolled by a rollpressing machine to obtain a density of approximately 2.3 g/cm³.Accordingly, the positive electrode sheet 220 (refer to FIG. 2) isobtained.

<Electrolyte Solution of Evaluation Battery>

Next, an electrolyte solution of the evaluation battery 100 will bedescribed. For the evaluation battery, the electrolyte solution may beobtained by mixing ethylene carbonate (EC), ethyl methyl carbonate(EMC), and dimethyl carbonate (DMC) at a ratio (molar ratio) of 3:3:4and then dissolving LiPF₆ in the mixture to a concentration of 1.1mol/L. In addition, an electrolyte solution can also be used into whichdifluorophosphate (LiPO₂F₂) and lithium bis(oxalato) borate (LiBOB) aredissolved either singularly or as a compound at a ratio of approximately0.05 mol/L.

<Fabrication of Evaluation Battery>

Next, more or less as shown in FIGS. 1 and 2, the evaluation battery 100is wound by overlapping the positive electrode sheet 220 and thenegative electrode sheet 240 fabricated as described above on top ofeach other and interposing the separators 262 and 264 between thepositive electrode sheet 220 and the negative electrode sheet 240.Subsequently, the wound electrode body 200 is fabricated by flatlydeforming the wound electrode body 200 in one direction that isperpendicular to the winding axis WL (refer to FIG. 2). In the woundelectrode body 200, the uncoated portion 222 of the positive electrodesheet 220 and the uncoated portion 242 of the negative electrode sheet240 are exposed on both sides of the separators 262 and 264.

In the evaluation battery 100, an opposite capacity ratio calculatedbased on a charging capacity of the positive electrode and a chargingcapacity of the negative electrode is adjusted to 1.5 to 1.9.

More or less as shown in FIG. 1, a battery case 300 of the evaluationbattery is a so-called square battery case and comprises a containermain body 320 and a lid 340. The battery case 300 comprises the bottomedand square tube-shape container main body 320 having a flat rectangularinner space as a space for housing the wound electrode body 200, and thelid 340 that blocks the opening of the container main body 320.Electrode terminals 420 and 440 are attached to the lid 340 of thebattery case 300. The uncoated portion 222 of the positive electrodesheet 220 of the wound electrode body 200 is connected to the electrodeterminal 420. The uncoated portion 242 of the negative electrode sheet240 of the wound electrode body 200 is connected to the electrodeterminal 440.

For this evaluation battery, the wound electrode body 200 attached tothe electrode terminals 420 and 440 of the lid 340 in this manner ishoused in the container main body 320. Subsequently, a joint 322 of thelid 340 and the container main body 320 of the battery case 300 iswelded by laser welding, an electrolyte solution is injected from aninlet 350 provided on the lid 340, and the inlet 350 is stopped.

<Conditioning>

Next, a conditioning process, a measurement of rated capacity, and SOCadjustment for the evaluation batteries 100 constructed as describedabove will be described in order.

In this case, the conditioning process is performed according toprocedures 1 and 2 below.

Procedure 1: After reaching 4.1 V by charging at a constant current of 1C, pause for 5 minutes.

Procedure 2: After Procedure 1, charge at a constant voltage for 1.5hours and subsequently pause for 5 minutes.

<Measurement of Rated Capacity>

Next, rated capacity of the evaluation test batteries is measuredaccording to procedures 1 to 3 below after the conditioning processdescribed above at a temperature of 25° C. and within a voltage range of3.0 V to 4.1 V.

Procedure 1: After reaching 3.0 V by discharging at a constant currentof 1 C, discharge at a constant voltage for 2 hours and subsequentlypause for 10 seconds.

Procedure 2: After reaching 4.1 V by charging at a constant current of 1C, charge at a constant voltage for 2.5 hours and subsequently pause for10 seconds.

Procedure 3: After reaching 3.0 V by discharging at a constant currentof 0.5 C, discharge at a constant voltage for 2 hours and subsequentlypause for 10 seconds.

Rated capacity: A discharge capacity (CCCV discharge capacity) ofdischarging from the constant current discharge to the constant voltagedischarge in Procedure 3 is adopted as the rated capacity. In thisevaluation batteries 100, the rated capacity reaches approximately 4 Ah.Reference Signs List

<SOC Adjustment>

SOC adjustment is performed according to Procedures 1 and 2 below. Inthis case, SOC adjustment may be performed after the conditioningprocess and the rated capacity measurement described above. In addition,in this case, SOC adjustment is performed under a temperatureenvironment of 25° C. in order to even out the effect of temperature.

Procedure 1: Charge at a constant current of 1 C from 3V to reach acharged state equivalent to approximately 60% of the rated capacity (SOC60%). Here, “SOC” refers to State of Charge.

Procedure 2: After Procedure 1, charge at a constant voltage for 2.5hours.

Accordingly, the evaluation battery 100 can be adjusted to apredetermined charged state.

A plurality of samples of the evaluation battery 100 which substantiallyonly differed from each other in the positive electrode active materialparticles 610 were prepared to comparatively evaluate the performance ofthe evaluation battery 100. In addition, in order to evaluate outputcharacteristics at a low temperature and a low charged state, “outputcharacteristics at −30° C. and a charged state of SOC 25%” and “outputcharacteristics at 0° C. and a charged state of SOC 25%” were evaluatedas characteristics of the evaluation battery 100.

<Output Characteristics at −30° C. and Charged State of SOC 25%>

Output characteristics at −30° C. and a charged state of SOC 25%(hereinafter referred to as “output characteristics 1” as appropriate)are obtained according to the following procedures.

Procedure 1 [SOC adjustment]: As SOC adjustment, charge at a constantcurrent of 1 C in a normal (in this case, 25° C.) temperatureenvironment in order to adjust to SOC 25%. Next, charge at a constantvoltage for 1 hour.

Procedure 2 [standing at −30° C. for 6 hours]: After Procedure 1 above,let the battery adjusted to SOC 25% stand in a −30° C.constant-temperature bath for 6 hours.

Procedure 3 [constant wattage discharge]: After Procedure 2 above,discharge at a constant wattage (W) from SOC 25% in a temperatureenvironment of −30° C. When doing so, measure the number of seconds fromthe start of discharge until voltage reaches 2.0 V.Procedure 4 [repetition]: Repeat Procedures 1 to 3 above while varyingthe condition of the constant wattage discharge voltage in Procedure 3from 80 W to 200 W. In this case, Procedures 1 to 3 are repeated whileincreasing the constant wattage discharge voltage in Procedure 3 in 10 Wincrements such that a first repetition is performed at 80 W, a secondrepetition is performed at 90 W, a third repetition is performed at 100W, and so forth until the constant wattage discharge voltage inProcedure 3 reaches 200 W. Here, the constant wattage discharge voltagein Procedure 3 is increased in 10 W increments. However, the constantwattage discharge voltage in Procedure 3 is not limited to the above,and the constant wattage discharge voltage may be increased inincrements of a certain number of watts (for example, in 5 W incrementsor in 15 W increments) or may be reduced from 500 W in decrements of acertain number of watts (for example, in 5 W decrements, 10 Wdecrements, or 15 W decrements).Procedure 5 [calculation of output characteristics 1]: For example, asshown in FIG. 10, plot the number of seconds until 2.0 V as measuredunder the constant wattage condition in Procedure 4 above on theabscissa and W at that point on the ordinate, and calculate W at 2seconds from an approximate curve of plots as the output characteristics1.

The output characteristics 1 represent an output that can be produced bythe evaluation battery 100 even in a case where the evaluation battery100 stands in an extremely low temperature environment of −30° C. for apredetermined period of time at a low amount of charge of around SOC25%. Therefore, the output characteristics 1 indicate that the higherthe value of W, the higher the output that can be produced by theevaluation battery 100. The output characteristics 1 also indicate thatthe higher the value of W, the more stable the output that may beobtained even at a low amount of charge such as around SOC 25%.

<Output Characteristics at 0° C. and Charged State of SOC 25%>

Output characteristics at 0° C. and a charged state of SOC 25%(hereinafter referred to as “output characteristics 2” as appropriate)are obtained according to the following procedures.

Procedure 1 [SOC adjustment]: As SOC adjustment, charge at a constantcurrent of 1 C in a normal (in this case, 25° C.) temperatureenvironment in order to adjust to SOC 25%. Next, charge at a constantvoltage for 1 hour.

Procedure 2 [standing at 0° C. for 6 hours]: After Procedure 1 above,let the battery adjusted to SOC 25% stand in a 0° C.constant-temperature bath for 6 hours.

Procedure 3 [constant wattage discharge]: After Procedure 2 above,discharge at a constant wattage (W) from SOC 25% in a temperatureenvironment of 0° C. When doing so, measure the number of seconds fromthe start of discharge until voltage reaches 2.0 V.

Procedure 4 [repetition]: Repeat Procedures 1 to 3 above while varyingthe condition of the constant wattage discharge voltage in Procedure 3from 350 W to 500 W. In this case, Procedures 1 to 3 are repeated whileincreasing the constant wattage discharge voltage in Procedure 3 in 10 Wincrements such that a first repetition is performed at 350 W, a secondrepetition is performed at 360 W, a third repetition is performed at 370W, and so forth until the constant wattage discharge voltage inProcedure 3 reaches 500 W. Here, the constant wattage discharge voltagein Procedure 3 is increased in 10 W increments. However, the constantwattage discharge voltage in Procedure 3 is not limited to the above,and the constant wattage discharge voltage may be increased inincrements of a certain number of watts (for example, in 5 W incrementsor in 15 W increments) or may be reduced from 500 W in decrements of acertain number of watts (for example, in 5 W decrements, 10 Wdecrements, or 15 W decrements).Procedure 5 [calculation of output characteristics 2]: For example, asshown in FIG. 11, plot the number of seconds until 2.0 V as measuredunder the constant wattage condition in Procedure 4 above on theabscissa and W at that point on the ordinate, and calculate W at 10seconds from an approximate curve of plots as the output characteristics2.

The output characteristics 2 represents an output that can be producedby the evaluation battery 100 in a case where the evaluation battery 100stands in a low temperature environment of 0° C. for a predeterminedperiod of time at a low amount of charge of around SOC 25%. The outputcharacteristics 2 indicate that the higher the value of W, the higherthe output that can be produced by the evaluation battery 100. Theoutput characteristics 2 also indicate that the higher the value of W,the more stable the output that may be obtained even at a low amount ofcharge of around SOC 25%.

Table 1 exemplifies, for a plurality of samples of the evaluationbattery 100 which substantially only differ from each other in thepositive electrode active material particles 610, a particle porosity ofthe positive electrode active material particles 610, a thickness of theshell portion 612, an average particle diameter (D50) of the positiveelectrode active material particles 610, output characteristics 1 of theevaluation battery 100, and output characteristics 2 of the evaluationbattery 100.

TABLE 1 Thickness Average particle diameter Presence/ Particle of shellof positive electrode active absence of −30° C., SOC 25%@2 s 0° C., SOC25%@10 s porosity portion material particles (D50) through hole Outputcharacteristics 1 Output characteristics 2 % μm μm — W W Sample 1 23.82.15 7.4 present 128 478 Sample 2 34.4 1.63 8.1 present 132 479 Sample 345.2 1.31 7.6 present 135 481 Sample 4 31.5 1.12 5.2 present 142 483Sample 5 27.6 0.98 4.1 present 143 484 Sample 6 59.8 0.89 7.6 present147 486 Sample 7 48.7 0.72 5.4 present 151 488 Sample 8 69.9 0.64 8.0present 152 489 Sample 9 77.4 0.36 6.1 present 155 491 Sample 10 80.90.23 3.8 present 158 493 Sample 11 90.2 0.13 5.3 present 161 494 Sample12 1.5 3.44 7.9 absent 79 411 Sample 13 2.8 2.56 6.2 absent 83 416Sample 14 19.3 1.69 6.1 absent 86 418 Sample 15 16.1 0.91 3.5 absent 90425 Sample 16 25.8 2.94 11.6 present 93 427

As shown in Table 1, there is a tendency that the higher the particleporosity of the positive electrode active material particles 610, thehigher the values of the output characteristics 1 and the outputcharacteristics 2. In addition, there is a tendency that the thinner thethickness of the shell portion 612 of the positive electrode activematerial particles 610, the higher the values of the outputcharacteristics 1 and the output characteristics 2. Furthermore, betweena case where the through hole 616 is present in the positive electrodeactive material particles 610 and a case where the through hole 616 isabsent therefrom, the values of the output characteristics 1 and theoutput characteristics 2 tend to be higher when the through hole 616 ispresent. The presence or absence of the through hole 616 may beconfirmed based on a sectional SEM image of the positive electrodeactive material particles 610 or a sectional SEM image of the positiveelectrode active material layer 223.

As described earlier, as shown in, for example, FIG. 1, the lithium-ionsecondary battery 100 comprises the positive electrode current collector221 and the porous positive electrode active material layer 223 retainedby the positive electrode current collector 221. In this case, as shownin FIG. 4, the positive electrode active material layer 223 contains thepositive electrode active material particles 610, the electricallyconductive material 620, and the binder 630.

In this lithium-ion secondary battery 100, as shown in, for example,FIG. 9, the positive electrode active material particles 610 may havethe shell portion 612 constituted by a lithium transition metal oxide,the hollow portion 614 formed inside the shell portion 612, and thethrough hole 616 penetrating the shell portion 612.

In the lithium-ion secondary battery 100, in the positive electrodeactive material layer 223 on average, a ratio of the hollow portion 614among an apparent sectional area of the positive electrode activematerial particles 610 is 23% or higher. In addition, the thickness ofthe shell portion 612 in the positive electrode active material layer223 on average is 2.2 μm or less.

In this case, on an arbitrary cross section of the positive electrodeactive material layer 223, a thickness T(k) of the shell portion 612 atan arbitrary position k on an inner surface of the shell portion 612 isdefined as a shortest distance from the arbitrary position on the innersurface of the shell portion 612 to an outer surface of the shellportion 612. The thickness of the shell portion 612 of the positiveelectrode active material particles 610 in the positive electrode activematerial layer 223 on average may be obtained by, for example, obtaininga thickness of the shell portion 612 of the positive electrode activematerial particles 610 on a plurality of arbitrary cross sections of thepositive electrode active material layer 223 and then determining anarithmetic average of the thickness of the shell portion 612 of thepositive electrode active material particles 610.

In this case, the arithmetic average converges by increasing the numberof cross sections of the positive electrode active material layer 223 onwhich the thickness of the shell portion 612 of the positive electrodeactive material particles 610 is calculated or by increasing the numberof arbitrary positions k on the inner surface of the shell portion 612at which the thickness T(k) of the shell portion 612 is obtained. Thethickness of the shell portion 612 being 2.2 μm or less in the positiveelectrode active material layer 223 on average means that the arithmeticaverage is 2.2 μm or less.

In the lithium-ion secondary battery 100, in the positive electrodeactive material layer 223 on average, a ratio of the hollow portion 614among an apparent sectional area of the positive electrode activematerial particles 610 is 23% or higher. In addition, the thickness ofthe shell portion 612 in the positive electrode active material layer223 on average is 2.2 μm or less. According to this lithium-ionsecondary battery, in the positive electrode active material layer 223on average, the ratio of the hollow portion 614 among the apparentsectional area of the positive electrode active material particles 610is 23% or higher, the positive electrode active material particles 610have a through hole 616 penetrating the shell portion 612, and thethickness of the shell portion 612 of the positive electrode activematerial particles 610 is extremely thin (in this case, 2.2 μm or less).Therefore, lithium ions diffuse rapidly into the shell portion (insideof the active material). As a result, the lithium-ion secondary batterycan stably produce high output even when the amount of charge is low.

In this lithium-ion secondary battery 100, there is a tendency that thehigher the particle porosity of the positive electrode active materialparticles 610, the greater the improvement in output characteristics.The particle porosity of the positive electrode active materialparticles 610 is favorably 30 or higher, more favorably 45 or higher,and even more favorably 60 or higher. Furthermore, in this lithium-ionsecondary battery 100, there is a tendency that the thinner the shellportion 612 of the positive electrode active material particles 610, thegreater the improvement in output characteristics. The thickness of theshell portion 612 of the positive electrode active material particles610 is more favorably 1.5 μm or less, even more favorably 1.00 μm orless, further more favorably 0.8 μm or less, and even further morefavorably 0.4 μm or less. Moreover, in order to secure durability of thepositive electrode active material particles 610 with respect to usage,the thickness of the shell portion 612 may be, for example, 0.05 μm ormore and, more favorably, 0.1 μm or more.

Moreover, the lithium transition metal oxide constituting the shellportion 612 of the positive electrode active material particles 610 maybe a compound which has a layered structure and which contains nickel asa constituent element. This lithium transition metal oxide may be, forexample, a compound which has a layered structure and which containsnickel, cobalt, and manganese as constituent elements. Alternatively,the lithium transition metal oxide may be a compound which has a layeredstructure and which is expressed asLi_(1+x)Ni_(y)Co_(z)Mn_((1-y-z))M_(γ)O₂, where 0≦x≦0.2, 0.1<y<0.9,0.1<z<0.4, M denotes an additive, and 0≦γ≦0.01. Furthermore, M as theadditive may be at least one additive selected from the group consistingof Zr, W, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, B, and F.

As described above, with this lithium-ion secondary battery 100, outputcharacteristics under low temperature are improved. In addition, stableoutput is obtained even when the amount of charge is low. Therefore, forexample, this lithium-ion secondary battery 100 is suitable as a highcapacity-lithium-ion secondary battery with a rated capacity of 3 Ah orhigher such as that used as a vehicle drive battery. Since thislithium-ion secondary battery 100 is capable of providing stable outputeven when the amount of charge is low, an SOC width during useincreases. As a result, an amount of power that can be extracted fromthe lithium-ion secondary battery 100 increases. As a vehicle drivebattery, this lithium-ion secondary battery 100 can extend mileage ofthe vehicle per charge.

Here, particularly favorably, the positive electrode active materialparticles 610 may be positive electrode active material particlesproduced by a production method comprising: a raw material hydroxideformation step of supplying ammonium ions to a transition metal solution(an aqueous solution of a transition metal compound), and precipitatingparticles of the transition metal hydroxide from the transition metalsolution; a mixing step of mixing the transition metal hydroxide with alithium compound to prepare an unfired mixture; and a calcining step ofcalcining the mixture to obtain the active material particles. In thiscase, the aqueous solution may contain at least one transition metalelement that composes the lithium transition metal oxide.

This raw material hydroxide formation step may include a nucleationstage in which the transition metal hydroxide is precipitated from thetransition metal solution and a particle growth stage in which thetransition metal hydroxide is grown in a state where a pH of thetransition metal solution is lowered from the nucleation stage.

In addition, in the raw material hydroxide formation step, the pH of thetransition metal solution in the nucleation stage may be 12 to 13, andthe pH of the transition metal solution in the particle growth stage maybe 11 or higher and less than 12. Accordingly, in the transition metalhydroxide to become a precursor of the positive electrode activematerial particles 610, transition metal hydroxide particles in which adensity of a vicinity of an outer surface is higher than an internaldensity can be obtained and the positive electrode active materialparticles 610 comprising a thin shell portion 612, a spacious hollowportion 614, and a through hole 616 can be obtained more stably.

At this point, furthermore, an ammonium ion concentration in thetransition metal solution in the nucleation stage may be 20 g/L orlower, and the ammonium ion concentration in the transition metalsolution in the particle growth stage may be 10 g/L or lower. Moreover,the ammonium ion concentration of the transition metal solution in thenucleation stage and the particle growth stage may be 3 g/L or higher.

In addition, this lithium-ion secondary battery 100 is characterized bythe positive electrode active material particles 610. Granules of activematerial particles are used as the positive electrode active materialparticles 610. In this case, as shown in FIG. 9, the granules of theactive material particles may comprise a shell portion 612 constitutedby a lithium transition metal oxide, a hollow portion 614 enclosed bythe shell portion 612, and a through hole 616 penetrating the shellportion 612. With the granules of these active material particles, inactive material particles 610 included in the granules on average, aratio of the hollow portion 614 among an apparent sectional area of theactive material particles 610 is 23% or higher, and a thickness of theshell portion 612 is 2.2 μm or less. In this case, on an arbitrary crosssection of the active material particles 610, a thickness of the shellportion 612 at an arbitrary position on an inner surface of the shellportion 612 is defined as a shortest distance from the arbitraryposition on the inner surface of the shell portion 612 to an outersurface of the shell portion 612.

In addition, in the active material particles 610 included in thegranules on average, the thickness of the shell portion may be 0.05 μmor more and, more favorably, 0.1 μm or more. Accordingly, sincedurability of the active material particles 610 is improved, theperformance of the lithium-ion secondary battery 100 can be stabilized.

As described above, the lithium transition metal oxide may be a compoundwhich has a layered structure and which contains nickel as a constituentelement. In addition, the lithium transition metal oxide may be acompound which has a layered structure and which contains nickel,cobalt, and manganese as constituent elements. Alternatively, thelithium transition metal oxide may be a compound which has a layeredstructure and which is expressed asLi_(1+x)Ni_(y)Co_(z)Mn_((1-y-z))M_(γ)O₂, where 0≦x≦0.2, 0.1<y<0.9,0.1<z<0.4, and M denotes an additive. Furthermore, M may be at least oneadditive selected from the group consisting of Zr, W, Mg, Ca, Na, Fe,Cr, Zn, Si, Sn, Al, B, and F.

Moreover, a production method of the active material particles 610comprises: a raw material hydroxide formation step of supplying ammoniumions to an aqueous solution of a transition metal compound, andprecipitating particles of the transition metal hydroxide from theaqueous solution; a mixing step of mixing the transition metal hydroxidewith a lithium compound to prepare an unfired mixture; and a calciningstep of calcining the mixture to obtain the active material particles610. In this case, the aqueous solution contains at least one transitionmetal element that composes the lithium transition metal oxide.

In addition, the raw material hydroxide formation step may include anucleation stage in which the transition metal hydroxide is precipitatedfrom the aqueous solution and a particle growth stage in which thetransition metal hydroxide is grown in a state where a pH of the aqueoussolution is lowered from the nucleation stage. Accordingly, activematerial particles 610 having a thin shell portion 612, a spacioushollow portion 614, and having a through hole 616 can be obtained in anefficient and stable manner.

A lithium-ion secondary battery, granules of active material particles,and a production method of the active material particles according to anembodiment of the present invention has been described above. However,the present invention is not limited to any of the embodiments above.

As described above, the present invention contributes to improvingoutput characteristics of a lithium-ion secondary battery. Therefore,the lithium-ion secondary battery according to the present invention ispreferable as a secondary battery used as a vehicle drive power supplyincluding a drive battery of a hybrid vehicle which is particularlyrequired to have high-rate output characteristics and high-level cyclingcharacteristics and drive batteries of a plug-in hybrid vehicle or anelectrical vehicle which are particularly required to have highcapacity. In this case, for example, as shown in FIG. 12, the secondarybattery can be preferably used as a vehicle drive battery 1000 fordriving a motor of a vehicle 1 such as an automobile in the form of anassembled battery in which a plurality of the secondary batteries areconnected in series. Particularly, the lithium-ion secondary batteryaccording to the present invention is capable of stably producing highoutput even when the amount of charge is low and can withstand use atlower amounts of charge. Therefore, the battery can be used efficientlyand, at the same time, the number of batteries used can be reduced andcost can be cut even when a high level of capacity is required. Asshown, the lithium-ion secondary battery 100 according to the presentinvention is particularly preferable as the vehicle drive battery 1000.

EXPLANATION OF REFERENCE NUMERALS

-   -   1 vehicle    -   100 lithium-ion secondary battery (evaluation battery)    -   200 wound electrode body    -   220 positive electrode sheet    -   221 positive electrode current collector    -   222 uncoated portion    -   223 positive electrode active material layer    -   224 intermediate portion    -   225 gap (cavities)    -   240 negative electrode    -   240 negative electrode sheet    -   241 negative electrode current collector    -   242 uncoated portion    -   243 negative electrode active material layer    -   245 gap (cavities)    -   262, 264 separator    -   280 electrolyte solution    -   290 charger    -   300 battery case    -   310, 312 gap    -   320 container main body    -   322 joint of lid and container main body    -   340 lid    -   350 inlet    -   352 sealing cap    -   360 safety valve    -   420 electrode terminal    -   420 a tip    -   440 electrode terminal    -   440 a tip    -   610 electrode active material particles    -   610 positive electrode active material particle    -   612 shell    -   612 a inner surface of the shell portion    -   614 hollow part    -   616 through hole    -   620 electrically conductive material    -   630 binder    -   710 negative electrode active material    -   730 binder    -   1000 vehicle drive battery    -   WL winding axis

The invention claimed is:
 1. A production method of active materialparticles, the method comprising: a raw material hydroxide formationstep of supplying ammonium ions to an aqueous solution of a transitionmetal compound to precipitate particles of a transition metal hydroxidefrom the aqueous solution, the aqueous solution containing at least onetransition metal element that composes a lithium transition metal oxide;a mixing step of mixing the transition metal hydroxide with a lithiumcompound to prepare an unfired mixture; and a calcining step ofcalcining the mixture to obtain active material particles, wherein theraw material hydroxide formation step includes: a nucleation stage inwhich the transition metal hydroxide is precipitated from the aqueoussolution; and a particle growth stage in which the transition metalhydroxide is grown in a state where a pH of the aqueous solution islowered from the nucleation stage, and wherein the aqueous solution hasa pH of 12 to 13 in the nucleation stage, and the aqueous solution has apH of 11 or higher and lower than 12 in the particle growth stage. 2.The production method of active material particles according to claim 1,wherein the aqueous solution has an ammonium ion concentration of 20 g/Lor lower in the nucleation stage, and the aqueous solution has anammonium ion concentration of 10 g/L or lower in the particle growthstage.
 3. The production method of active material particles accordingto claim 2, wherein the aqueous solution has an ammonium ionconcentration of 3 g/L or higher in the nucleation stage and theparticle growth stage.